The present invention relates to a method of metal treatment. More particularly the invention relates to a method of improving corrosion resistance of a metal. The method has particular utility when the metal is to be subsequently painted, or operations such as bonding rubber to metals or bonding metals to metals are to be carried out subsequent to the silane treatment. The method comprises applying a solution containing one or more amino silanes in admixture with one or more multi-silyl-functional silanes to a metal substrate in order to form a corrosion resistant coating. The method is particularly suitable for use on cold-rolled steel, zinc, iron, aluminium and aluminium alloy surfaces.
Most metals are susceptible to some form of corrosion, in particular atmospheric corrosion including the formation of various types of rust. Such corrosion may significantly affect the quality of such metal substrates, as well as that of the products produced therefrom. Although corrosion may often be removed from the metal substrates, these processes are often, time consuming costly and may further diminish the integrity of the metal. Additionally, where polymer coatings such as paints, adhesives or rubbers are applied to the metal substrates, corrosion of the base metal material may cause a loss of adhesion between the polymer coating and the base metal. Such a loss of adhesion between a coating layer and the base metal may likewise lead to corrosion of the metal.
Metallic coated steel sheet such as galvanized steel for example is used in many industries, including the automotive, construction and appliance industries. In most cases, the galvanized steel is painted or otherwise coated with a polymer layer to achieve a durable and aesthetically-pleasing product. Galvanized steel, particularly hot-dipped galvanized steel, however, often develops xe2x80x9cwhite rustxe2x80x9d during storage and shipment. White rust (also called xe2x80x9cstorage stainxe2x80x9d) is typically caused by moisture condensation on the surface of the galvanized steel which reacts with the zinc coating. White rust is aesthetically unappealing and impairs the ability of the galvanized steel to undergo subsequent process steps such as being painted or otherwise coated with a polymer. Thus, prior to such coating, the zinc surface of the galvanized steel must be pretreated in order to remove the white rust which is present, and prevent it from reforming beneath the polymer layer. Various methods are currently employed to not only prevent the formation of white rust during shipment and storage, but also to prevent the formation of the white rust beneath a polymer coating (e.g., paint).
It is well established that prevention of the formation of white rust on hot-dipped galvanized steel during storage and shipping can be achieved by treating the surface of the steel with a thin chromate film. While such chromate coatings do provide resistance to the formation of white rust, chromium is highly toxic and environmentally undesirable.
It is also known to employ a phosphate conversion coating in conjunction with a chromate rinse in order to improve paint adherence and provide corrosion protection. It is believed that the chromate rinse covers the pores in the phosphate coating, thereby improving the corrosion resistance and adhesion performance. Once again, however, it is highly desirable to eliminate the use of chromate altogether. Unfortunately, however, the phosphate conversion coating is generally not effective without the chromate rinse.
Aluminium alloys are particularly susceptible to corrosion as the alloying elements used to improve the metal""s mechanical properties (e.g., copper, magnesium and zinc) will decrease corrosion resistance.
Recently, various techniques for eliminating the use of chromate have been proposed. These include the steps of providing an aqueous alkaline solution comprising an inorganic silicate and a metal salt in an amount to coat a steel sheet, followed by treating the silicate coating with an organofunctional silane (U.S. Pat. No. 5,108,793).
U.S. Pat. No. 5,292,549 teaches the rinsing of metal sheet with an aqueous solution containing low concentrations of an organofunctional silane and a cross linking agent in order to provide temporary corrosion protection. The cross-linking agent cross-links the organofunctional silane to form a denser siloxane film. The ratio range of silane to cross-linker is 20:1-2:1.
WO 98/30735 discloses a method of preventing corrosion using 2 treatment solutions, applied separately. The first solution employs a multi-silyl-functional silane cross-linker while the second solution employs an organofunctional silane.
U.S. Pat. No. 5,433,976 teaches the rinsing of a metal sheet with an alkaline solution containing a dissolved silicate or aluminate, an organofunctional silane and a cross-linking agent in order to form an insoluble composite layer containing siloxane.
WO 98/19798 relates to a method of preventing corrosion of metal sheet effected by the application of a solution containing one or more hydrolyzed vinyl silanes to the metal sheet. The method is particularly useful as a pretreatment step prior to painting of galvanized steel as the vinyl functionalities promote the adhesion between the metal surface and the paint coating. A disadvantage, however, is that the vinyl silanes do not bond particularly well to the metal surface.
U.S. Pat. No. Re. 34, 675 (re-issue of U.S. Pat. No. 4,689,085) describes coupling agent and primer compositions which comprise a conventional silane coupling agent and bis (trialkoxy) organo compound, and partially hydrolyzed products of such mixtures.
It is an object of the present invention to provide a method of providing long-term corrosion resistance to a metal substrate.
It is another object of the present invention to provide a method of providing a coating for long-term corrosion resistance to a metal substrate sheet which essentially employs a single-step treatment process.
It is a further object of the present invention to provide a treatment solution for providing a coating for corrosion resistance to metal substrate, wherein the treatment composition need not be removed prior to painting.
It is a further object of the present invention to provide a treatment coating and solution for promoting rubber to metal bonding.
It is a further object of the present invention to provide a treatment solution for promoting metal to metal bonding using adhesives.
The foregoing objects may be accomplished, in accordance with one aspect of the present invention, by providing a method of improving corrosion resistance of a metal substrate, comprising of the steps of:
(a) providing a metal substrate, the said metal substrate chosen from the group consisting of:
steel;
steel coated with a metal chosen from the group consisting of: zinc, zinc alloy, aluminium and aluminium alloy;
iron;
zinc and zinc alloys;
aluminium; and
aluminium alloy; and
(b) applying a long-term coating on the metal substrate by contacting the metal substrate with a solution containing one or more hydrolyzed or partially hydrolyzed amino silanes, one or more hydrolyzed or partially hydrolyzed multi-silyl-functional silanes and a solvent and substantially removing the solvent.
The applicants have found that corrosion resistance of metal, particularly cold-rolled steel, steel coated with a metal chosen from the group consisting of zinc, zinc alloy, aluminium and aluminium and aluminium alloy, aluminium and aluminium alloy per se and iron, can be improved by applying a treatment solution containing one or more hydrolyzed or partially hydrolyzed amino silanes to said metal, wherein the treatment solution additionally contains one or more multi-silyl-functional silanes, having either 2 or 3 trisubstituted silyl groups, wherein the multi-silyl-functional silane(s) has been at least partially hydrolyzed. The treatment solution forms a long-term corrosion resistant coating upon curing.
The provision of a coating for long-term corrosion resistance is surprisingly superior to conventional chromate based treatments, and avoids the chromium disposal problem. In addition, the coating provides superior adhesion of the metal substrate to paint, rubber, adhesive or other polymer layers.
In addition to the above mentioned corrosion preventative properties of the treatment method of the present application, the applicant has also found that the above mentioned coatings exhibit particular utility in the promotion of rubber to metal bonding and metal to metal bonding using adhesives.
The treatment methods of the present invention may be used on any of a variety of metal substrates including particularly cold-rolled steel, steel coated with a metal chosen from the group consisting of zinc, zinc alloy, aluminium and aluminium and aluminium alloy, aluminium and aluminium alloy per se, and iron. The method of the present invention is effected by applying a treatment solution containing one or more hydrolyzed or partially hydrolyzed amino silanes to said metal, wherein the treatment solution additionally contains one or more multi-silyl-functional silanes having either 2 or 3 trisubstituted silyl groups to the metal, wherein the multi-silyl-functional silane(s) has been at least partially hydrolyzed.
As used herein, the term xe2x80x9csubstitutedxe2x80x9d aliphatic or aromatic means an aliphatic or aromatic group wherein the carbon backbone may have a heteroatom located within the backbone or a heteroatom or heteroatom containing group attached to the carbor backbone.
The preferred amino silanes which may be employed in the present invention each have a single trisubstituted silyl group, wherein the substituents are individually chosen from the group consisting of alkoxy, acyloxy and aryloxy. Thus, the amino silanes which maybe used in the present invention may have the general structure 
R is chosen from the group consisting of hydrogen, C1-C24 alkyl, preferably C1-C6 alkyl, C2-C24 acyl, preferably C2-C4 acyl, and each R may be the same or different. Preferably R is individually chosen from the group consisting of hydrogen, ethyl, methyl, propyl, iso-propyl, butyl, iso-butyl, sec-butyl ter-butyl and acetyl.
X is a group selected from the group consisting of a bond, a substituted or unsubstituted aliphatic or aromatic group. Preferably X is selected from the group chosen from the group consisting of a bond, C1-C6 alkylene, C1-C6 alkenylene, C1-C6 alkylene substituted with at least one amino group, C1-C6 alkenylene substituted with at least one amino group, arylene and alkylarylene
R1 is a group individually selected from the group consisting of hydrogen, C1-C6 alkyl, C1-C6 alkenyl, C1-C6 alkyl substituted with at least one amino group, C1-C6 alkenyl substituted with at least one amino group, arylene and alkylarylene. Preferably R1 is individually selected from the group consisting of hydrogen, ethyl, methyl, propyl, iso-propyl, butyl, iso-butyl, sec-butyl ter-butyl and acetyl.
The particular preferred amino silane employed in the method of the present invention is xcex3-aminopropyltriethoxysilane, which will be referred to as xcex3-APS, and having the structure: 
More than one multi-silyl-functional silane may be employed in the treatment solution. Each multi-silyl-functional silane has at least 2 trisubstituted silyl groups, wherein the substituents are individually chosen from the group consisting of alkoxy and acyloxy. Preferably the multi-silyl-functional silane of the present invention has the general structure. 
wherein Z is selected from the group consisting of either a bond, an aliphatic or aromatic group; each R3 is an alkyl or acyl group, and n is 2 or 3.
Each R3 is chosen from the group consisting of hydrogen, C1-C24 alkyl, preferably C1-C6 alkyl, C2-C24 acyl, preferably C2-C4 acyl, and may be the same or different. Preferably each R3 is individually selected from the group consisting of ethyl, methyl, propyl, iso-propyl, butyl, iso-butyl, sec-butyl, ter-butyl and acetyl.
Preferably Z is selected from the group consisting of a bond, C1-C6 alkylene, C1-C6 alkenylene, C1-C6 alkylene substituted with at least one amino group, C1-C6 alkenylene substituted with at least one amino group, arylene and alkylarylene. In the case where Z is a bond, the multi-functional silane comprises two trisubstituted silyl groups which are bonded directly to one another.
The preferred multi-silyl-functional silane is 1,2-bis-(triethoxysilyl)ethane , referred to as BTSE and having the structure: 
Other suitable multi-functional silanes include 1,2-bis-(trimethoxysilyl)ethane (TMSE), and 1,6-bis-(trialkoxysilyl)hexanes (including 1,6-bis-(trimethoxysilyl)hexanes), 1,2-bis-(triethoxysilyl)ethylene, 1,4-bis-(trimethoxysilylethyl)benzene, and bis-(trimethoxysilylpropyl)amine.
The above-described amino and multi-functional silanes must be at least partially, and preferably fully hydrolyzed so that the silanes will bond to the metal surface. During hydrolysis, the alkyl or acyl groups (i.e., the xe2x80x9cRxe2x80x9d and xe2x80x9cR3xe2x80x9d, moieties) are replaced with a hydrogen atom. As used herein, the term xe2x80x9cpartially hydrolyzedxe2x80x9d simply means that only a portion of the alkyl or acyl groups on the silane have been replaced with a hydrogen atom. The silanes should preferably be hydrolyzed to the extent that at least two of the alkyl or acyl groups on each molecule have been replaced with a hydrogen atom. Hydrolysis of the silanes may be accomplished merely be mixing the silanes with water, and optionally including a solvent such as an alcohol in order to improve solubility.
One significant advantage of the present invention is that the treatment solution may be applied directly onto the surface of the metal without the need for an underlying layer of silicates, aluminate or other coating. Another significant advantage is the utility and convenience to the user of a one step treatment.
The present invention is particularly suitable if, subsequent to treatment of the metal substrate being carried out, the metal substrate is to be painted or coated with a polymer such as an adhesive or rubber. This may take place after one or more silane treatments, and advantageously after curing of said silane treatment(s).
The pH of the solution is also preferably maintained below about 7, and, most preferably between about 3 and about 6, in order to improve hydrolysis. The pH may be adjusted, for example, by the addition of an acid, such as acetic, oxalic, formic or propionic acid. If the pH is permitted to increase above about 7, the hydrolyzed multi-functional silane may begin to polymerize via a condensation reaction. If this is permitted to occur, the corrosion resistance will be significantly reduced since the silane may not bond strongly to the metal surface.
The concentration of multi-silyl-functional silanes such as BTSE in the solution should be between about 0.01% and about 10%, preferably greater than 0.1%. More preferably, a concentration of between about 0.4% and about 3%, most preferably about 2% is preferred.
The concentration of amino silanes in the solution should be between about 0.01 and 10%. More preferably, a concentration of between about 0.2% and about 2%, most preferably about 1% is preferred.
The ratio between the amino silanes and the multi-silyl-functional silanes is essential to the efficacy of the present invention in providing long-term corrosion resitance. The term xe2x80x9clong-termxe2x80x9d as used herein is relative to xe2x80x9ctemporary corrosion protectionxe2x80x9d coating, such as that disclosed in the patent U.S. Pat. No. 5,292,549, in which it claimed xe2x80x9cthe siloxane film may be removed by rinsing the metallic coated steel sheet in an alkaline solution prior to coating the sheet with a phosphate conversion coating and a paint.xe2x80x9d In the context of corrosion resistance xe2x80x9clong-termxe2x80x9d means a coating which resists being washed off or removed. The present invention shows superior properties on metal surface and resists being removed by alkaline solution. This aspect can be assessed by using an alkaline rinse solution, as set out in Example 9, to try to remove the coatings of the present invention. The ratios of amino silanes and the multi-silyl-functional silanes used in the present invention are in the range 4:1-1:8, preferably 2:1-1:4, more preferably a ratio of greater than 1:2.
Although a more concentrated solution will provide a greater film thickness on the metal, this comes at the expense of increased cost. In addition, thicker films are often weak and brittle. The film thickness is generally in the range of 0.05-0.2 xcexcm.
It should be noted that the concentration of silanes discussed and claimed herein are all measured in terms of the ratio between the amount of unhydrolyzed, multi-silyl-functional silanes employed (i.e., prior to hydrolyzation), and the total volume of treatment solution components (i.e., silanes, water, optional solvents and pH adjusting acids). In addition, the concentrations refer to the total amount of unhydrolyzed multi-silyl-functional silanes added, as multiple silanes may optionally be employed in this treatment solution.
The solution temperature is not critical. Temperatures down to 0C should be satisfactory. There is no need to heat the solution but a temperature of between 15 and 60xc2x0 C. for the treatment bath during treatment will be satisfactory. Higher temperatures may cause polymerisation of the silane (i.e. they may shorten the bath life) and will have no benefit.
Since the solubility in water of some of the silanes used may be limited, the treatment solution may optionally include one or more solvents, such as alcohols, in order to improve silane solubility. The alcohol may also improve the stability of the treatment solution, as well as the wettability of the metal substrate. The use of alcohols or other non-aqueous solvents such as acetone is also particularly usefufer metal substrates which re prone to corrosion upon contact with water (such as galvanic corrosion of certain alloys, including CRS). Particularly preferred alcohols include: methanol, ethanol, propanol, butanol and isomers thereof. The amount employed will depend upon the solubility of the particular multi-silyl-functional silanes in the treatment solution and thus the concentration range of alcohol to water in the treatment solution of the present invention is in the ratio of 1:99 to 99:1, (by volume). There should be sufficient water to ensure at least partial hydrolysis of the silane, and thus it is preferable that at least 5 parts of water be employed for every 95 parts of alcohol. Alcohols may, however, be omitted entirely if the silane(s) is soluble in water. When alcohols are employed, methanol and ethanol are the preferred alcohols.
Preparation of the treatment solution itself is straightforward. The unhydrolyzed amino silanes are prehydrolyzed by diluting with water to obtain a desired concentration. The pH may be adjusted using an acid as described above. The BTSE is prehydrolyzed by using a similar method and the solutions are mixed and the pH be adjusted using acid. Alcohol may optionally be employed to aid solubility or stability as required. In practice the baths will be replenished with the silanes utilised in the invention. These may be supplied pre-hydrolyzed and pre-mixed as a water dilutable concentrate.
The metal substrate to be treated is preferably solvent and/or alkaline cleaned (by techniques well-known in the prior art) prior to application of the above-described treatment composition of the present invention. The treatment solution may then be applied to the cleaned metal by either dipping the metal into the solution (also referred to as xe2x80x9crinsingxe2x80x9d), spraying the solution onto the surface of the metal, or even wiping or brushing the treatment solution onto the metal substrate Indeed any method which leaves a substantially even film on theserface may effectively be employed. When the preferred application method of dipping is employed, the duration of dipping is not critical, as it will generally not affect the resulting film thickness. It is preferred that the dipping time be between about 2 seconds and about 50 minutes, preferably between about 0.5 minutes and 2 minutes to ensure complete coating of the metal.
If the metal is not to be coated with a polymer such as paint, and particularly in the case of aluminium and aluminium alloys, the silane coating should advantageously be cured following the application process described above. Curing will polymerize the hydrolyzed silanol groups. The metal may be blown dry or dried in place.
The silane treatment coating may be cured at a temperature of between about 40xc2x0 C. and 180xc2x0 C. The curing time is dependant upon the curing temperature although this time is not crucial. It is sufficient just to dry the article in the shortest possible time. Lower temperatures would excessively lengthen drying times. After curing, a second treatment solution may be applied or the first treatment solution may be reapplied, and cured if desired. Curing times may be between 0.5 minutes and 1 hour but preferably a curing period of between about 0.5 minutes and 3 minutes is used. Curing will eventually take place even at room temperatures over a sufficient period of time. The metal substrate may be painted immediately but this is not essential.